Microseismic analysis of the geological strata around the bore of fluid injection and production wells is typically effected by the use of seismic sensor assemblies (sondes), mounted downhole in the area of the fluid flow. Usually a number of sondes are mounted in the well at different levels in the bore. Deployment techniques have been developed to allow the sensors to become almost completely mechanically decoupled from the flow induced noise from the tubing.
Systems for permanently installing a sonde against an inner wall of a pipe, such as the casing of a fluid extraction well, are known. Such systems are described in, for example, U.S. Pat. Nos. 5,092,423, 5,181,565, 5,200,581, 5,111,903, 6,289,985, 6,173,804 and 5,318,129.
Typically, a sonde comprises a clamp which permanently or semi-permanently engages with the inner casing of a well. For example, the clamp may be lowered into the well in a retracted state and then once in position activated to engage with the well casing using a pressure actuated system, which may use external pressure sources or well pressure. Such a clamp is described in patent application no. EP-A-1370891, the contents of which are incorporated herein by reference, which describes C-shaped ring clamps. It is also possible to activate the clamp near the top of the well, and simply drag it down the well, acting against friction between the clamp and well casing, into the desired position.
A disadvantage of these systems is that because the sondes (for example with C-shaped ring clamps) are released from the tubing and clamped to the inside of the casing, any large tubing movement, i.e. typically more than 15 cm, can cause the risk of coupling the sondes back to the tubing. Such movement is invariably axial or rotational. These systems only perform at their best when the tubing movement is small. Small movements can also be accommodated by the wires from the sensors mounted on the casing and running up the tubing, whereas large movements will result in breakage of these wires.
Well completions differ significantly from well to well and temperature changes cause thermal expansion to the installed tubing. Completions have to be designed to allow for the tubing axial or rotational movement, and this can be done by the installation of a seal bore packer for example.
For well completions of this type where tubing movement occurs, it would therefore be preferable to provide a means of allowing the sondes to move along the inside of the casing when the tubing moves while maintaining good mechanical decoupling.
With such an arrangement, the sondes must be able to move along the axis of the borehole when the tubing moves, this tubing movement being possible in either direction. Therefore, the sonde must be secured to the tubing by some mechanical means which must have the following properties:    a) it is strong enough to allow the sonde to be dragged along the casing;    b) it does not change the frequency properties of the sonde by changing or adding unwanted resonance; and    c) most importantly, it does not provide a path for flow noise from tubing to sonde.
It is an object of the present invention to provide a sonde having such securement means.